Engineering Materials 2E VOLUME1 Episode 1 ppt

The Yield Strength, Tensile Strength, Hardness and Ductility definitions, stress-strain curves true and nominal, testing methods, data 9.. Fast fracture, toughness and fatigue where th

S E C O N D E D I T I O N

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Engineering Materials 1

A n lntroduction to their Properties and Applications

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Introduction to Dislocations, 3rd Edition

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British Library Cataloguing in Publication Data

Ashby, Michael E

Engineering materials 1 an introduction to their

properties and applications - 2nd ed

Engineering materials 1 an introduction to their properties and

applicationsby Michael F Ashby and David R H Jones - 2nd ed

p cm

Rev.ed of Engineering materials 1980

Includes bibliographical references and index

ISBN 0 7506 3081 7

1 Materials I Jones, David R H (David Rayner Hunkin),

1945- 11 Ashby, M.F Engineering materials III Title

TA403.A69 96-1677 620.1’1-dc20 CIP

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General introduction

1 Engineering Materials and their Properties

examples of structures and devices showing how we select the right

material for the job

3

A Price and availability

2 The Price and Availability of Materials 15

what governs the prices of engineering materials, how long will supplies

last, and how can we make the most of the resources that we have?

B The elastic moduli

stress and strain; Hooke’s Law; measuring Young’s modulus; data for

design

the types of bonds that hold materials together; why some bonds are

stiff and others floppy

how atoms are packed in crystals - crystal structures, plane (Miller)

indices, direction indices; how atoms are packed in polymers, ceramics

and glasses

6 The Physical Basis of Young’s Modulus 58

how the modulus is governed by bond stiffness and atomic packing; the

glass transition temperature in rubbers; designing stiff materials -

man-made composites

the mirror for a big telescope; a stiff beam of minimum weight; a stiff

beam of minimum cost

vi Contents

C Yield strength, tensile strength, hardness and ductility

8 The Yield Strength, Tensile Strength, Hardness and Ductility

definitions, stress-strain curves (true and nominal), testing methods,

data

9 Dislocations and Yielding in Crystals

the ideal strength; dislocations (screw and edge) and how they move to

give plastic flow

10 Strengthening Methods and Plasticity of Polycrystals

solid solution hardening; precipitate and dispersion strengthening;

work-hardening; yield in polycrystals

11 Continuum Aspects of Plastic Flow

the shear yield strength; plastic instability; the formability of metals and

polymers

12 Case Studies in Yield-limited Design

materials for springs; a pressure vessel of minimum weight; a pressure

vessel of minimum cost; how metals are rolled into sheet

D Fast fracture, toughness and fatigue

where the energy comes from for catastrophic crack growth; the

condition for fast fracture; data for toughness and fracture toughness

13 Fast Fracture and Toughness

14 Micromechanisms of Fast Fracture

ductile tearing, cleavage; composites, alloys - and why structures are

more likely to fail in the winter

15 Fatigue Failure

fatigue testing, Basquin’s Law, Coffin-Manson Law; crack growth rates

for pre-cracked materials; mechanisms of fatigue

16 Case Studies in Fast Fracture and Fatigue Failure

fast fracture of an ammonia tank; how to stop a pressure vessel blowing

up; is cracked cast iron safe?

E Creep deformation and fracture

high-temperature behaviour of materials; creep testing and creep curves;

consequences of creep; creep damage and creep fracture

17 Creep and Creep Fracture

Contents vii

Arrhenius's Law; Fick's first law derived from statistical mechanics of

thermally activated atoms; how diffusion takes place in solids

metals and ceramics - dislocation creep, diffusion creep; creep in

polymers; designing creep-resistant materials

20 The Turbine Blade - A Case Study in Creep-limited Design 197

requirements of a turbine-blade material; nickel-based super-alloys,

blade cooling; a new generation of materials? - metal-matrix composites,

ceramics, cost effectiveness

F Oxidation and corrosion

21 Oxidation of Materials

the driving force for oxidation; rates of oxidation, mechanisms of

oxidation; data

22 Case Studies in Dry Oxidation

making stainless alloys; protecting turbine blades

23 Wet Corrosion of Materials

voltages as driving forces; rates of corrosion; why selective attack is

especially dangerous

24 Case Studies in Wet Corrosion

how to protect an underground pipeline; materials for a light-weight

factory roof; how to make motor-car exhausts last longer

G Friction, abrasion and wear

25 Friction and Wear

surfaces in contact; how the laws of friction are explained by the

asperity-contact model; coefficients of friction; lubrication; the adhesive

and abrasive wear of materials

26 Case Studies in Friction and Wear

the design of a journal bearing; materials for skis and sledge runners;

viii Contents

Final case study

27 Materials and Energy in Car Design

the selection and economics of materials for automobiles

Appendix 1 Examples

Appendix 2 Aids and Demonstrations

Appendix 3 Symbols and Formulae

to select materials which best fit the demands of the design - economic and aesthetic demands, as well as demands of strength and durability The designer must understand the properties of materials, and their limitations

This book gives a broad introduction to these properties and limitations It cannot make you a materials expert, but it can teach you how to make a sensible choice of material, how to avoid the mistakes that have led to embarrassment or tragedy in the past, and where to turn for further, more detailed, help

You will notice from the Contents list that the chapters are arranged in groups, each group describing a particular class of properties: the elastic modulus; the fracture toughness; resistance to corrosion; and so forth Each such group of chapters starts by

defining the property, describing how it is measured, and giving a table of data that we use

to solve problems involving the selection and use of materials We then move on to the

basic science that underlies each property, and show how we can use this fundamental knowledge to design materials with better properties Each group ends with a chapter

of case studies in which the basic understanding and the data for each property are applied to practical engineering problems involving materials Each chapter has a list

of books for further reuding, ranked so that the more elementary come first

At the end of the book you will find sets of examples; each example is meant to consolidate or develop a particular point covered in the text Try to do the examples that derive from a particular chapter whilesthis is still fresh in your mind In this way you will gain confidence that you are on top of the subject

No engineer attempts to learn or remember tables or lists of data for material

properties But you should try to remember the broad orders-of-magnitude of these quantities All grocers know that ’a kg of apples is about 10 apples’ - they still weigh them, but their knowledge prevents them making silly mistakes which might cost them money In the same way, an engineer should know that ’most elastic moduli lie between

1 and lo3 GN m-2; and are around 102GN mW2 for metals’ - in any real design you need

an accurate value, which you can get from suppliers’ specifications; but an order-of-

2 Engineering Materials 1

magnitude knowledge prevents you getting the units wrong, or making other silly, and possibly expensive, mistakes To help you in this, we have added at the end of the book

a list of the important definitions and formulae that you should know, or should be able

to derive, and a summary of the orders-of-magnitude of materials properties

To the lecturer

This book is a course in Engineering Materials for engineering students with no previous background in the subject It is designed to link up with the teaching of Design, Mechanics and Structures, and to meet the needs of engineering students in the 1990s for a first materials course, emphasising applications

The text is deliberately concise Each chapter is designed to cover the content of one 50-minute lecture, twenty-seven in all, and allows time for demonstrations and illustrative slides A list of the slides, and a description of the demonstrations that we have found appropriate to each lecture, are given in Appendix 2 The text contains sets

of worked case studies (Chapters 7, 12, 16, 20, 22, 24, 26 and 27) which apply the material of the preceding block of lectures There are examples for the student at the end of the book; worked solutions are available separately from the publisher

We have made every effort to keep the mathematical analysis as simple as possible while still retaining the essential physical understanding, and still arriving at results which, although approximate, are useful But we have avoided mere description: most

of the case studies and examples involve analysis, and the use of data, to arrive at numerical solutions to real or postulated problems This level of analysis, and these data, are of the type that would be used in a preliminary study for the selection of a material or the analysis of a design (or design-failure) It is worth emphasising to students that the next step would be a detailed analysis, using more precise mechanics

(from the texts given as 'further reading') and data from the supplier of the material or from in-house testing Materials data are notoriously variable Approximate tabulations like those given here, though useful, should never be used for final designs

Chapter 1

Engineering materials and their properties

Introduction

There are, it is said, more than 50,000 materials available to the engineer In designing

a structure or device, how is the engineer to choose from this vast menu the material which best suits the purpose? Mistakes can cause disasters During World War 11, one class of welded merchant ship suffered heavy losses, not by enemy attack, but by breaking in half at sea: the fracture toughness of the steel - and, particularly, of the welds was too low More recently, three Comet aircraft were lost before it was realised that the design called for a fatigue strength that - given the design of the window frames - was greater than that possessed by the material You yourself will be familiar with poorly- designed appliances made of plastic: their excessive 'give' is because the designer did not allow for the low modulus of the polymer These bulk properties are listed in Table 1.1, along with other common classes of property that the designer must consider when choosing a material Many of these properties will be unfamiliar to you - we will introduce them through examples in this chapter They form the basis of this first course on materials

In this first course, we shall also encounter the classes of materials shown in Table 1.2 More engineering components are made of metals and alloys than of any other class of solid But increasingly, polymers are replacing metals because they offer a combination

of properties which are more attractive to the designer And if you've been reading the newspaper, you will know that the new ceramics, at present under development world wide, are an emerging class of engineering material which may permit more efficient heat engines, sharper knives, and bearings with lower friction The engineer can combine the best properties of these materials to make composites (the most familiar is fibreglass) which offer specially attractive packages of properties And - finally - one should not ignore natural maferials like wood and leather which have properties which

- even with the innovations of today's materials scientists - are hard to beat

In this chapter we illustrate, using a variety of examples, how the designer selects materials so that they provide him or her with the properties needed As a first example, consider the selection of materials for a

Plastic-handled screwdriver

A typical screwdriver has a shaft and blade made of a high-carbon steel, a metal Steel

is chosen because its modulus is high The modulus measures the resistance of the material to elastic deflection or bending If you made the shaft out of a polymer like polyethylene instead, it would twist far too much A high modulus is one criterion in

4 Engineering Materials 1

Table 1.1 Classes of property Economic

General Physical Mechanical

Thermal

Electrical and Magnetic

Environmental Interaction

Production

Aesthetic

Price and availability Recyclability Density Modulus Yield and tensile strength Hardness

Fracture toughness Fatigue strength Creep strength Damping Thermal conductivity Specific heat

Thermal expansion coefficient Resistivity

Dielectric constant Magnetic permeability Oxidation

Corrosion Wear Ease of manufacture Joining

Finishing Colour Texture Feel

the selection of a material for this application But it is not the only one The shaft must have a high yield strength If it does not, it will bend or twist if you turn it hard (bad screwdrivers do) And the blade must have a high hardness, otherwise it will be damaged by the head of the screw Finally, the material of the shaft and blade must not only do all these things, it must also resist fracture - glass, for instance, has a high modulus, yield strength and hardness, but it would not be a good choice for this application because it is so brittle More precisely, it has a very low fracfure toughness That of the steel is high, meaning that it gives a bit before it breaks

The handle of the screwdriver is made of a polymer or plastic, in this instance polymethylmethacrylate, otherwise known as PMMA, plexiglass or perspex The handle has a much larger section than the shaft, so its twisting, and thus its modulus,

is less important You could not make it satisfactorily out of a soft rubber (another polymer) because its modulus is much too low, although a thin skin of rubber might be useful because its friction coefficient is high, making it easy to grip Traditionally, of course, tool handles were made of another natural, polymer - wood - and, if you measure importance by the volume consumed per year, wood is still by far the most important polymer available to the engineer Wood has been replaced by PMMA